1. Field of the Invention
The present invention relates to a process for preparing a polyhydroxystyrene derivative, and more particularly to a process for preparing polyhydroxystyrene derivatives by using an alkaline alcohol solution as the solvent to dissolve polyhydroxystyrene in order to allow the polyhydroxystyrene to react with an ester in a homogeneous system.
2. Description of the Prior Art
A typical photoresist composition includes a resin, a photoacid generator, an inhibitor, and other additives. An ideal photoresist composition should meet the following requirements such as the good resolution, photo-sensitivity, etching resistance, adhesion, thermal stability, film uniformity and the processibility. Since the resin is the major component in a photoresist, it has important influence on the above-mentioned properties, and particularly, it is the key factor for the enhanced ability of the etching resistance, adhesion, and film uniformity.
Generally speaking, since copolymers of polyhydroxystyrene have the low absorption at a wavelength of the 248 nm and the good thermal stability, they are often used as the resin components in a photoresist. Generally, there are two methods to prepare copolymers (or derivatives) of polyhydroxystyrene. One is the direct polymerization of two different monomers. For example, U.S. Pat. No. 4,775,730 disclosed the direct polymerization of the styrene derivative with another alkene derivative, and then the resultant copolymer was then subsequently hydrolyzed to produce the desired product. The reaction is shown in formula (III): ##STR1##
J. Polym. Sci.: Part A; Polym. Chem. Ed. 1986, 24, 2971, however, disclosed that two different styrene monomers are polymerized and a functional group is then selectively decomposed. The reaction is shown in formula (IV): ##STR2##
Another way to prepare a polyhydroxystyrene copolymer involves using the polyhydroxystyrene as a starting material to allow polyhydroxystyrene reacting with an ester reagent. For example, in European Patent 0634696, polyhydroxystyrene is dissolved in tetrahydrofuran (THF), potassium t-butoxide KOC(CH.sub.3).sub.3 ! is then added, and then the mixture is allowed to react with di(t-butyl)dicarbonate to obtain a polyhydroxystyrene derivative. However, under such conditions, the potassium salt of the polyhydroxystyrene is not dissolved in THF. Therefore, the subsequent reaction of polyhydroxystyrene with di(t-butyl)dicarbonate is conducted in a non-homogeneous system. The overall reaction is as shown in formula (V): ##STR3##
In a brief, the conventional processes for preparing a polyhydroxystyrene derivative either involve a complicated synthetic procedure or are rather time-consuming, and since the reaction is conducted in a non-homogeneous system, the functionalized extent of each polyhydroxystyrene molecule differs greatly, thus the quality of the product is not easily controlled.